Re: [PATCH] pc-bios: s390x: Save iplb location in lowcore

[David Hildenbrand]

On 04.03.20 09:59, Janosch Frank wrote:
> On 3/3/20 5:13 PM, David Hildenbrand wrote:
>> On 03.03.20 16:50, Janosch Frank wrote:
>>> The POP states that for a list directed IPL the IPLB is stored into
>>> memory by the machine loader and its address is stored at offset 0x14
>>> of the lowcore.
>>>
>>> ZIPL currently uses the address in offset 0x14 to access the IPLB and
>>> acquire flags about secure boot. If the IPLB address points into
>>> memory which has an unsupported mix of flags set, ZIPL will panic
>>> instead of booting the OS.
>>>
>>> As the lowcore can have quite a high entropy for a guest that did drop
>>> out of protected mode (i.e. rebooted) we encountered the ZIPL panic
>>> quite often.
>>
>> How did this ever work? Or does this only become relevant with secure boot?
> 
> I'd guess that until secure boot ZIPL never touched this and with it we
> never hit the right combination of flags to trigger a ZIPL panic.
> 
> This way of getting to the IPLB was used before diag308 was available,
> i.e. way before KVM got to IBM Z. It looks like ZIPL only uses it for
> secure boot for some reason and hence we never implemented it before.
> 
> I'm also in discussion with the ZIPL developers to make this more robust.
> 

Thanks for the clarification!

[...]

> 
> Do you want to add a patch or shall I add it to my cleanup series?

Would be great if you could add a cleanup.

-- 
Thanks,

David / dhildenb